1. Field of the Invention
The present invention relates to a particle moving type display device which moves charged particles to thereby perform display.
2. Description of the Related Art
In recent years, a particle moving type display device has been proposed which moves charged particles to thereby perform display. As one example of the particle moving type display device, there is a device which disperses charged migration particles in an insulating liquid, applies an electric field to the charged migration particles to thereby move the charged migration particles, and accordingly performs display.
Moreover, as an electric migration display element for use in the electric migration display device, an element has been proposed in U.S. Pat. No. 3,892,568 comprising: a pair of substrates disposed at a predetermined gap; a dispersion medium which is disposed in the gap between the substrates and in which the charged migration particles are dispersed; and a pair of electrodes arranged in the vicinity of the dispersion medium.
FIG. 10 is a diagram showing a schematic constitution of a conventional electric migration display element. A space held between a pair of transparent electrodes 22a, 22b formed on a pair of transparent substrates 21a, 21b, respectively, is charged with a liquid medium 25 in which particles 23, 24 having different optical reflection characteristics (optical characteristics) and charge polarities (charging characteristics) are dispersed.
Moreover, when voltages are applied to the respective transparent electrodes 22a, 22b from a power supply 26 via a switch 27 capable of switching the polarity, the particles 23, 24 are selectively collected in the vicinity of the respective electrodes to thereby perform display.
Furthermore, as another conventional example of an electric migration display element, as described in U.S. Pat. No. 6,639,580, a reflection plate is disposed on one substrate 21b. Moreover, for example, when the liquid medium 25 is charged with black particles to perform black display, the black particles are moved toward one substrate 21b. On the other hand, to perform white display, the black particles are moved sideways, incident light is reflected by the reflection plate, and the reflected light from the reflection plate is directly visually recognized.
However, in this conventional electric migration display device, layers of the particles 23, 24 having different optical characteristics are superimposed on each other in a vertical direction in a case where the electric migration display device (electric migration display element) is seen, for example, from above one transparent substrate 21a. Therefore, to obtain sufficient brightness (contrast), an amount of the particles 23, 24 needs to be sufficiently increased in such a manner to make a lower particle layer invisible.
If the particles have a property of completely reflecting or absorbing the light, the light is completely screened by the particles, and does not leak to the lower layer. However, in reality, even the particles having any property transmit the light to a certain degree. Therefore, the light reaches the particles of the lower layer, and reflects there, and the lower-layer particles are seen through.
Especially, white particles of the following materials are used, but there is not any particle having a great light screening effect in the present situations. When a light screening ratio (ratio of transmitted light intensity with respect to incident light intensity) of white particles is small, a large part of the light leaking to the lower layer is absorbed by black particles. As a result, bright white display is not obtained.
Therefore, when the black particles 24 and the white particles 23 are used as particles having different optical characteristics, an amount of the white particles 23 needs to be increased in order to obtain sufficiently bright white display. However, when the amount of the white particles 23 is increased in this manner, a charged particle amount in the liquid medium (dispersion liquid) increases, and therefore an electric field screening effect by the charged particles at a voltage application time increases. Therefore, when the electric field screening effect increases in this manner, a response speed of the electric migration display element drops in a case where a driving voltage is equal. Therefore, the driving voltage needs to rise in order to increase the response speed. That is, in a case where the amount of the white particles 23 is increased in order to obtain sufficient brightness (contrast), the driving voltage needs to be increased in order to compensate for the drop of the response speed.
On the other hand, in a case where the reflected light from the reflection plate is directly visually recognized to thereby perform white display, with a usual reflection plate (regular reflection plate) having a low directivity, the sufficient brightness (contrast) cannot be obtained, and the display glares. Therefore, a diffusion reflection plate having a high directivity, or a directive reflection plate has to be used as the reflection plate, and there is a problem that costs increase.
The present invention has been developed in view of this present situation, and one of the objects is to provide a particle moving type display device in which bright display is possible without increasing the amount of the charged particles, or the costs.